[Extensions] Add metrics for UserScriptListener

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index e32dc38380278985d11dc2f22e52acd876372746..7db1d6f041792d61bdcf7b953b4fc4d65f9e4c9d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9877,6 +9877,26 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.ThrottledNetworkRequestDelay" units="milliseconds">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The duration for which a network request was delayed while waiting for

[Mark P, 2015/03/19 20:19:01]

nit: for->by

[Devlin, 2015/03/19 21:09:42]

Done.

+    extension user script load. Recorded once per resource that is delayed, at

[Mark P, 2015/03/19 20:19:01]

resource == request?

[Devlin, 2015/03/19 21:09:42]

Done.

+    the time the resource resumes loading.
+  </summary>
+</histogram>
+
+<histogram name="Extensions.ThrottledNetworkRequests" units="requests">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The total number of network requests that were delayed by extension user
+    script load. Recorded once (per profile) when all the profile's extensions'
+    user scripts have loaded.

[Mark P, 2015/03/19 20:19:01]

If a user adds a extension with user script (that thus needs to load), does that
cause a new emit?  Or is this effectively an on-profile-open emit (load all
extensions' user scripts on profile open)?

[Devlin, 2015/03/19 21:09:42]

Initially, I thought it was on-profile-load, but it turns out it could restrict
any time a user script is loading.  Corrected.

+  </summary>
+</histogram>
+
 <histogram name="Extensions.ToolstripLoadTime" units="milliseconds">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>Time taken to load a toolstrip.</summary>